NAT Traversal for Media Conferencing

ABSTRACT

Methods for establishing a direct peer-to-peer (“P2P”) connection between two computers are disclosed. In particular, the methods are designed to work in cases where one or both of the computers are connected to a private network, such private networks being interconnected via a public network, such as the Internet. The connections between the private network and the public network are facilitated by network address translation (“NAT”).

This is a divisional application of, and claims priority to, U.S. patentapplication Ser. No. 10/769,841 filed on Feb. 2, 2004 and which ishereby incorporated by reference.

BACKGROUND

The invention relates generally to computer systems and moreparticularly, but not by way of limitation, to a technique forestablishing a peer-to-peer (“P2P”) connection between two computers inthe presence of network address translation (“NAT”). Establishing a P2Pconnection between two computers is useful for the implementation ofvarious applications, including, for example, gaming, file sharing, andmedia (audio, video, etc.) conferencing. Although the system herein isdescribed with reference to Internet Protocol (“IP”) networks, theinvention is not so limited and could be used with other network types.

Large public networks, such as the Internet, frequently have connectionsto smaller private networks, such as those maintained by a corporation,Internet service provider, or even individual households. By their verynature, public networks must have a commonly agreed upon allocation ofnetwork addresses, i.e., public addresses. For a variety of reasons,some of which are discussed in more detail below, maintainers of privatenetworks often choose to use private network addresses for the privatenetworks that are not part of the commonly agreed upon allocation. Thus,for network traffic from the private network to be able to traverse thepublic network, some form of NAT is required.

As is known to those skilled in the art, the basic principle of NAT isthat a private network, having a private addressing scheme, may beconnected to a public network, having a standardized addressing scheme,e.g., the Internet through a network address translator. A networkaddress translator (details of which are known to those skilled in theart) alters the data packets being sent out of the private network tocomply with the addressing scheme of the public network. Particularly,the network address translator replaces the originating private addressand port number of a packet with its own public address and an assignedport number. A network address translator also alters the data packetsbeing received for computers on the private network to replace thedestination public address and port number with the correct privateaddress and port number of the intended recipient. As used herein, theterm address should be construed to include both an address and a portnumber if appropriate in the context, as would be understood by one ofordinary skill in the art.

NAT has become increasingly common in modern network computing. Oneadvantage of NAT is that it slows the depletion of public networkaddress space. For example, TCP/IP addressing, which is used on theInternet, comprises four strings of three digits each, thus providing afinite address space. Additionally, certain portions of this addressspace are reserved for particular uses or users, further depleting theactual number of addresses available. However, if NAT is used, a privatenetwork or subnet may use an arbitrary number of addresses, and stillpresent only a single, standardized public address to the outside world.This makes the number of available addresses practically limitless,because each private network could, theoretically, use exactly the sameprivate addresses.

Another advantage provided by NAT is increased security. The increasedsecurity arises in part from the fact that those on the public networkcannot determine the actual (i.e., private) network address of acomputer on a private network. This is because only the public addressis provided on the public network by the network address translator.Additionally, this public address may correspond to any number ofcomputers on the private network. This feature also facilitates networkaddress translators acting as firewalls, because data received by thenetwork address translator that does not correspond to a request from acomputer on the private network may be discarded.

While this security works well in conventional client-server computing,where connections to a “server” on the public network are initiated by a“client” on the private network, it poses problems for P2P connections.In many P2P applications, it is desirable to establish a connectiondirectly between two computers (i.e., peers) that would be consideredclients in a traditional sense, but that may act both as clients and asservers in the context of the P2P connection. Establishing a directconnection becomes increasingly difficult if one or both of the peers islocated behind one or more levels of NAT.

Historically, there have been various techniques for establishing a P2Pconnection in the presence of NAT. These techniques include Relaying,Connection Reversal, UDP Hole Punching, UDP Port Number Prediction, andSimultaneous TCP Connection Initiation. Each of these techniques suffersfrom various deficiencies that render them undesirable for variousapplications. For example, Relaying increases network overhead andlatency, which is undesirable for timing critical applications such asvideo conferencing or gaming. Connection Reversal will only work if onlyone of the peers is located behind a network address translator. UDPHole Punching, as the name implies, works well only with UDP connectionsand is less successful using other transport layer protocols, such asTCP. UDP Port Number Prediction requires predictable behavior by thevarious components, and is also geared toward UDP connections.Simultaneous TCP Connection Initiation requires a degree of luck, bothwith regard to addressing and port assignment and connection timing,resulting in a fragility that renders it unsuitable for generalapplication.

Thus, it would be beneficial to provide a means to permit computers eachlocated behind one or more NAT layers to establish a direct, P2Pconnection in a way that is efficient, reliable, and requires minimalredesign of existing network infrastructure.

SUMMARY

The present invention relates to establishing a direct P2P connectionbetween computers wherein one or both of the computers are locatedbehind one or more layers of NAT. In one embodiment, an initiatingcomputer sends a message to the receiving computer by way of a Relayserver. The Relay server retransmits this request to the receivingcomputer, which has already established a connection with the Relayserver. Upon receiving this request, the receiving computer determines alist of addresses on which it believes it can be contacted and transmitsthis information to the Relay server, which then re-transmits theinformation to the initiating computer. Upon receiving this information,the receiving computer sends direct initiation messages to the addressesprovided by the receiving computer until a direct P2P connection isestablished. If a connection cannot be established, the initiatingcomputer sends a message so indicating to the Relay server, whichre-transmits this information to the receiving computer. The receivingcomputer then starts the process over, this time acting as theinitiating computer.

In another embodiment, both the initiating computer and the receivingcomputer have logged into a Relay server. An initiating computerdetermines its public address by querying an address-determinationserver. It then generates a list of addresses on which it believes itcan be contacted and transmits this information to the Relay server,which re-transmits this information to the receiving computer. Onreceiving the initiating computer's address information, the receivingcomputer also determines its public address and generates a list ofaddresses on which it believes it can be contacted. The receivingcomputer also sends a sequence of messages to the initiating computer,which are discarded by a network address translator behind which theinitiating computer is located, but which set up the receivingcomputer's network address translator to later accept a connection. Thereceiving computer then transmits the addresses on which it believes itcan be contacted to the Relay server, which re-transmits thisinformation to the initiating computer. The initiating computer, havingreceived this information, sends a sequence of initiation messagesdirectly to the addresses provided by the receiving computer, and thesemessages are able to pass through the receiving computer's networkaddress translator because of the earlier sequence of messages sent bythe receiving computer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a network topology in which the teachings of the presentinvention may be used.

FIG. 2 illustrates the address header information in connection withvarious types of network address translation.

FIG. 3 illustrates an operational matrix for selecting which techniquein accordance with the present invention is necessary to establish aconnection between two peers.

FIG. 4 illustrates the sequence of messages in one technique ofestablishing a peer-to-peer connection in accordance with the presentinvention.

FIG. 5 illustrates the sequence of messages in another technique ofestablishing a peer-to-peer connection in accordance with the presentinvention.

DETAILED DESCRIPTION

Techniques (methods and devices) to establish a peer-to-peer (“P2P”)connection between computers each located behind one or more levels ofnetwork address translation (“NAT”) are described herein. The followingembodiments of the invention, described in terms applications compatiblewith computer systems manufactured by Apple Computer, Inc. of Cupertino,Calif., are illustrative only and should not be considered limiting inany respect.

Turning now to FIG. 1, a general network topology in which the presentinvention may be used is illustrated. A plurality of “client” or “peer”computers 110 a through 110 f are interconnected by public network 150,which could be, for example, the Internet. Peer computers 110 c and 110e are directly connected to public network 150. Peer computers 110 d and110 f are connected behind network address translators 123 and 124.Still other peer computers 110 a and 110 b are connected behind twolayers or levels of network address translators, 121 and 122. Relayserver 130, which is used in the relaying technique of the prior art isalso connected to public network 150. Finally, Address Determinationserver 140, used to provide a peer's public address in accordance withthe teachings of the present invention, is also connected to publicnetwork 150. Address Determination server 140 provides the means for apeer to determine its public address and port number as assigned by anetwork address translator behind which the peer resides.

With reference now to FIG. 2, the address headers used in connectionwith three common types of NAT are illustrated. A first type of NAT is afull cone NAT, which means that all requests from the same internal(private) IP address and port are mapped to the same external (public)IP address and port. In this type of NAT, any external host can send apacket to the internal host by sending the packet to the mapped externaladdress and host. Address header information 210 for a full cone NATthus includes internal IP address 211, internal port assignment 212, andexternal port assignment 213. (The external address will be the publicaddress of the network address translator.)

A second type of NAT is a restricted cone NAT, which means that allrequests from the same internal IP address and port are mapped to thesame external IP address and port. However, unlike a full cone NAT, anexternal host can send a packet to the internal host only if theinternal host had previously sent a packet to the IP address of theexternal host. Thus data header 220 includes the same information aswith a full cone NAT (internal IP address 221, internal port assignment222, and external port assignment 223) and also includes additionalfield 224 that identifies the remote address to which a packet haspreviously been sent.

A third type of NAT is a port-restricted cone NAT, which is a restrictedcone NAT further restricted to port numbers. Specifically, an externalhost can send a packet to an internal host only if the internal host hadpreviously sent a packet to the specific IP address and port from whichthe “return” packet originated. Thus, to successfully traverse a portrestricted cone NAT, address header 230 must include all the informationin a restricted cone header (i.e., internal IP address 231, internalport assignment 232, external port assignment 233, remote IP address234) and the additional information of the remote port to which a packethas previously been sent.

Additionally, a network address translator may also use port addresstranslation (“PAT”). When PAT is used, the network address translatorwill use a different port for each outbound address/port combination. IfPAT is not implemented, a single port number is used for each client.

FIG. 3 illustrates an operational matrix for determining which processin accordance with the present invention may be used for establish a P2Pconnection between two peers. For purposes of the following discussion,it is assumed that PEER-0 is initiating the connection with PEER-1. Ascan be seen from FIG. 3, if both PEER-0 and PEER-1 are located on thepublic network, as with peers 110 c and 110 e of FIG. 1, then no specialtechnique is needed as each computer's packets indicate its true addressand port number. If PEER-0, located on the public network, attempts toinitiate a connection with PEER-1 located on a private network, as withpeer 110 c attempting to initiate a connection with peer 110 d, then afirst technique ALPHA, discussed below in connection with FIG. 4 may beused. In the situation where both PEER-0 and PEER-1 are both located onprivate networks, as with peer 110 a attempting to initiate a connectionwith peer 110 d, the technique ALPHA will work if one of the peers isbehind a full-cone network address translator (described above withreference to FIG. 2). Otherwise, a second technique BETA, discussedbelow in connection with FIG. 5 must be used. Technique BETA will workfor establishing a connection between two peers each located on aprivate network, even if both peers are located behind multiple NATlayers, as with peers 110 a and 110 b of FIG. 1.

Turning now to FIG. 4, first process ALPHA for establishing a connectionbetween initiating peer 110 c located on public network 150 andreceiving peer 110 d located behind network address translator 123 isillustrated. A P2P connection can only be established with peer 110 d ifthis peer has previously logged in to some third party server, forexample, Relay server 130. Thus peer 110 d transmits login message 431to Relay server 130. Initiating peer 110 c must also login to Relayserver 130, by transmitting login message 421. Provided that both peers110 c and 110 d have logged into Relay server 130, initiating peer 110 cmay then request a connection with peer 110 d by sending connectionrequest message 422 to Relay server 130, which acts as an intermediary.Relay server 130 then transmits this information to peer 110 d asnotification message 441.

Upon receiving notification message 441 from Relay server 130 that aconnection is requested, peer 110 d transmits address-determinationmessage 432 to Address Determination server 140. The purpose of thismessage is solely for allowing peer 110 d to determine its public IPaddress and port number assignment, which are assigned by networkaddress translator 123. Address Determination server 140 returns replymessage 411 to peer 110 d, from which peer 110 d can determine itspublic IP address and port assignment.

Peer 110 d, having determined its port assignment generates IP-List 433,which is a list of private IP addresses and corresponding ports on whichpeer 110 d can receive a connection. IP-List 433 also includes FlippedList, which is the binary complement of the listing of IP address andport combinations. It will be recognized that a Flipped-List may begenerated in accordance with any user-specified transformation of theIP-List's contents (a binary complement is but one transformation). Theflipped list is used because some network address translatorsinterrogate outgoing packets and, if they find a local (private) IPaddress, convert it to the network address translator's external(public) IP address. This action would destroy IP-List 433, which isused by peer 110 c for initiating the connection. Once generated,IP-List 433 is transmitted to Relay server 130, which re-transmits theIP-List 442 to peer 110 c.

Taking the information from IP-list 442, peer 110 c then beginsinitiating a direct P2P connection with peer 110 d. Specifically, peer110 c steps through the addresses and ports contained in IP-list 442issuing connection requests 423 to peer 110 d until it is able toestablish a connection with peer 110 d (using the “flipped” aspect of IPList 442 if necessary). In one embodiment, session initiation protocol(“SIP”) invitation messages comprise connection requests 423. Generally,connection request messages 423 after issued one after another, with aspecified time delay between each transmission. For example, three (3)seconds. If peer 110 c is unable to establish a connection with peer 110d, peer 110 c can ascertain that peer 110 d is behind a network addresstranslator and/or firewall and that network address translator and/orfirewall is restricted. Peer 110 c would then contact Relay server 130and ask the Relay server to have peer 110 d contact peer 110 c. At thattime, the process of FIG. 4 is repeated, but with peer 110 d trying toinitiate communication.

Turning now to FIG. 5, process BETA is illustrated which worksregardless of which peer initiates communication and regardless of howmany network address translators or firewalls either or both peers arebehind. For purposes of explanation of FIG. 5, it is assumed that peer110 f is initiating a P2P connection with peer 110 d. The beginning ofthe process requires both peers to have logged onto Relay server 130,which is done by the transmission of logon message 521 by peer 110 f andlogon message 531 by peer 110 d. To initiate a P2P session, peer 110 ftransmits address detection message 522 to Address Detection server 140.

As with method ALPHA described above, address determination message 522allows peer 110 f to determine its public IP address and port number.Address Determination server 140 returns message 511 to peer 110 f, fromwhich peer 110 f can determine its public IP address and portassignment. Having determined its IP address and port assignment, peer110 f then generates IP-List 523, which is a list of local IP addressesand corresponding ports. As with method ALPHA described above, IP-List523 also includes a flipped List, i.e., the binary complement of thelisting of IP address and port combinations. Once generated, IP-List 523is transmitted to peer 110 d by way of Relay server 130 as part of callmessage 524.

Peer 110 d, upon receiving peer 110 fs IP-list 523 as part of relayedcall message 541 from Relay server 130, then sends its own addressdetermination message 531 to Address Determination server 140 so thatpeer it may determine its public IP address and port number. AddressDetermination server 140 returns message 512 to peer 110 d in which itspublic IP address and port assignments are identified. Peer 110 d thenbegins sending a series of I-Ping messages 532 to peer 110 f. In oneembodiment, each I-Ping message 532 is a junk UDP packet, and one issent to each of peer 110 fs IP-List entries. While each I-Ping packet isdropped by peer 110 fs network address translator, these packets set uppeer 110 d's network address translator to later receive and accept anincoming connection request message (e.g., a SIP invitation message).

Having determined its IP address and port assignment, peer 110 d thengenerates its own IP-List 533, which also includes a flipped list. Oncegenerated, IP-List 533 is transmitted to peer 110 f by way of Relayserver 130 as message 534. Relay server 130 re-transmits thisinformation via message 542 to peer 110 f, which determines the IPaddress/port pairs in unflip operation 525. Peer 110 f then sends aconnection request message 526 (e.g., a SIP invitation message) to eachIP/port pair in peer 110 d's IP-List until a connection is received andaccepted. Once one of the connection request messages is accepted, a P2Pconnection is established directly between peer 110 f and peer 110 d. Asdescribed above, individual connection request messages may be staggeredin time such as, by three (3) seconds. As previously noted, process BETAdescribed above may be used with all types of connections becauseneither peer knows or cares if its targeted system is public or privateor how many layers of NAT are present.

While the invention has been disclosed with respect to a limited numberof embodiments, numerous modifications and variations will beappreciated by those skilled in the art. It is intended that all suchvariations and modifications fall with in the scope of the followingclaims.

1. A method for initiating a peer-to-peer network connection between afirst computer and a second computer across a public network, wherein atleast one of the first and second computers is on a private network andis connected to the public network through a network address translator,the first and second computers having each established a connection to acommon Relay server, the method comprising: receiving at the secondcomputer from the Relay server a connection request messagere-transmitted from the first computer; sending an address-determinationmessage from the second computer to an address-determination server;receiving at the second computer a reply to the address-determinationmessage; composing and sending from the second computer to the Relayserver for re-transmitting to the first computer a message comprisingone or more addresses corresponding to the second computer; and waitingfor an attempt by the first computer to initiate a direct connectionbetween the first computer and the second computer.
 2. The method ofclaim 1 further comprising: receiving at the second computer a messagefrom the Relay server originating at the first computer indicating thatthe first computer is unable to establish a direct connection with thesecond computer; and attempting to initiate a connection directly fromthe second computer to the first computer in response to the messageindicating that the first computer is unable to establish a directconnection with the second computer.
 3. The method of claim 2 whereinthe act of attempting to initiate a connection directly from the secondcomputer to the first computer comprises: sending a connection requestmessage from the second computer to the Relay server for re-transmittingto the first computer; receiving at the second computer from the Relayserver a message originating at the first computer, said messagecomprising one or more addresses corresponding to the first computer;and attempting to initiate a connection from the second computerdirectly to one or more of the addresses corresponding to the firstcomputer.
 4. The method of claim 3 wherein the one or more addressescorresponding to the first computer comprises one or more IP addressesand one or more port numbers.
 5. The method of claim 4 wherein themessage comprising one or more addresses corresponding to the firstcomputer further comprises a specified transformation of the one or moreaddresses.
 6. The method of claim 5 wherein the specified transformationcomprises a binary complement transformation.
 7. The method of claim 1wherein the Address Determination server is a server other than thecommon Relay server.
 8. A non-transitory program storage device havinginstructions stored therein for causing a processor to initiate apeer-to-peer network connection between a first computer and a secondcomputer across a public network, wherein at least one of the first andsecond computers is on a private network and is connected to the publicnetwork through a network address translator, the first and secondcomputers having each established a connection to a common Relay server,said instructions comprising: instructions to receive at the secondcomputer, from the Relay server, a connection request messagere-transmitted from the first computer; instructions to transmit anaddress-determination message from the second computer, to anaddress-determination server; instructions to receive, at the secondcomputer, a reply to the address-determination message; instructions tocompose and send from the second computer to the Relay server forre-transmitting to the first computer, a message comprising one or moreaddresses corresponding to the second computer; and instructions to waitfor an attempt by the first computer to initiate a direct connectionbetween the first computer and the second computer.
 9. Thenon-transitory program storage device of claim 8 further comprising:instructions to receive at the second computer a message from the Relayserver originating at the first computer indicating that the firstcomputer is unable to establish a direct connection with the secondcomputer; and instructions to attempt to initiate a connection directlyfrom the second computer to the first computer in response to themessage indicating that the first computer is unable to establish adirect connection with the second computer.
 10. The non-transitoryprogram storage device of claim 9 wherein the instructions to attempt toinitiate a connection directly from the second computer to the firstcomputer comprises: instructions to send a connection request messagefrom the second computer to the Relay server for re-transmitting to thefirst computer; instructions to receive at the second computer from theRelay server a message originating at the first computer, said messagecomprising one or more addresses corresponding to the first computer;and instructions to attempt to initiate a connection from the secondcomputer directly to one or more of the addresses corresponding to thefirst computer.
 11. The non-transitory program storage device of claim10 wherein the one or more addresses corresponding to the first computercomprises one or more IP addresses and one or more port numbers.
 12. Thenon-transitory program storage device of claim 11 wherein the messagecomprising one or more addresses corresponding to the first computerfurther comprises a specified transformation of the one or moreaddresses.
 13. The non-transitory program storage device of claim 12wherein the specified transformation comprises a binary complementtransformation.
 14. The non-transitory program storage device of claim 8wherein the Address Determination server is a server other than thecommon Relay server.
 15. A device, comprising: a network interfaceadapted to couple to a network; memory for storing instructions inaccordance with claim 8; and a processor for executing the instructions.